The present invention relates to apparatus for exposing at least one face of a panel, in particular a printed circuit panel.
Such apparatuses are used for manufacturing printed circuits from a panel coated in a photosensitive material that is to have artwork placed in front of it bearing the pattern of tracks to be generated on the printed circuit. A light beam passes through the artwork and thus serves to expose the panel.
Such exposure apparatuses are known, e.g. from European patents EP 618 505, EP 807 505, and EP 807 856, in which the light source and the panel to be exposed are both stationary and exposure is performed over the entire surface that is to be exposed without scanning.
However, in that type of apparatus where the entire surface is exposed simultaneously, the chemical reactivity of the photosensitive material is not optimum. The efficiency of the reaction is improved when exposure is stronger and instantaneous exposure time shorter.
Such apparatuses are also known in which the surface of the panel is exposed by scanning, with a light beam coming from a light source being reflected on a rotary mirror.
Unfortunately, the definition with which tracks are drawn, and the fineness of such tracks is directly related to the angle of incidence of the light beam on the artwork. Each light beam occupies a circular cone about an axis that is inclined at a greater or lesser angle relative to the surface that is to be exposed, and that is referred to as declination. The half-angle at the apex of the cone represents collimation, i.e. the degree of parallelism between light rays. It will thus be understood that the angle of incidence of a light beam depends both on its collimation and on its declination. Consequently, when the light is not collimated and/or when some of the light beams reach the surface for exposure at an angle of incidence that is too large, the size of the tracks and the paths they follow are generally modified relative to the artwork.
Similarly, when exposure is not performed uniformly, then tracks are formed unequally and the quality of the resulting printed circuit is poor.
The object of the invention is to provide an exposure apparatus that makes it possible to improve exposure of a surface by scanning, in particular for manufacturing printed circuits, by providing a strip of light which presents both a good uniformity and a good angle of incidence relative to the panel.
Such apparatus makes it possible to make printed circuits having a conductor track density that is very high, and it ensures that the tracks are very fine and follow very accurate paths, i.e. it ensures that the surface for exposure is perfectly exposed. The size of the conductor tracks of printed circuits that are to be made using such apparatus lies in the range 25 micrometers (xcexcm) to 50 xcexcm, and they are spaced apart by approximately the same amount.
It will thus be understood that a light beam reaching the surface to be exposed at a poor angle of declination produces a parallax error causing the light to be offset on the surface to be exposed, thereby shifting the conductor tracks away from the design location. The same applies to the size of the tracks which increases and becomes less precise with worsening collimation of the beam relative to the surface to be exposed. Under extreme circumstances, those two phenomena taken together can lead to short circuits because tracks touch each other.
In addition, the quality of development, i.e. the quality of removal of that portion of the photosensitive material which is not to overlie future tracks during etching, and consequently the quality of that etching, depends on prior transformation of the photosensitive material, which transformation is itself related to the quantity of light energy received. It will thus be understood that when the light beam is not uniform, it gives rise to non-uniform transformation of the photosensitive material and thus to tracks following paths that are imprecise and in extreme cases this can lead to tracks being interrupted.
Throughout the description below, the term xe2x80x9clight stripxe2x80x9d is used to designate the set of light beams reaching the surface to be exposed, and the term xe2x80x9cmean angle of incidencexe2x80x9d corresponds to the angle measured in any plane substantially transversal to the plane of the surface to be exposed and within which half of the light flux reaches the surface, the other half reaching the surface at any angle.
Each light beam lies in a circular cone of axis substantially perpendicular to the plane of the surface to be exposed. Under such circumstances, the half-angle at the apex which represents collimation is less than or equal to the mean angle of incidence.
Since the length of the light strip is not less than the length of one of the sides of the panel to be exposed, it suffices to scan in a single direction in order to expose the entire surface of the panel. The light strip is moved relative to the surface to be exposed (or vice versa) in a direction extending transversely to the length of the light strip so as to scan the entire surface. The direction in which the panel is scanned corresponds to the direction of one of the sides of the panel. Thus, by generating a light strip in a first direction parallel to one of the sides of the panel, scanning is performed in a second direction substantially transversal to said first direction.
In addition, since the zone that is illuminated during exposure forms a strip, scanning time is reduced by using a narrow light pencil of flux density greater than when exposing the entire surface. For equal flux density, the greater the height of the quadrilateral constituted by the light strip, the shorter the scanning time. However, the greater the height of the quadrilateral, the more difficult it is to obtain light at a small angle of incidence which is collimated and uniform over the entire quadrilateral, and the lower the flux density. It is therefore necessary to find a compromise.
In addition, it must be possible to guarantee identical exposure throughout scanning, i.e. that the uniformity and the angle of incidence of the light beam remain constant throughout displacement of the strip of light.
In a first aspect, the present invention provides apparatus for improving the exposure of a surface, in particular for manufacturing printed circuits, by processing a light beam emitted from a single stationary light source.
In this first aspect, the invention provides apparatus comprising:
means for holding at least one artwork and said panel on a frame;
an optical system comprising a light source emitting a light beam, processor means for processing said light beam to generate a uniform and collimated light beam having a mean angle of incidence relative to the surface to be exposed of less than 2xc2x0 and having illumination uniformity that departs by less than xc2x110% from a mean value, and shaper means enabling said uniform and collimated light beam to be transformed into a uniform and collimated light strip on the surface of the panel to be exposed and including said artwork, said uniform and collimated light strip being of length not less than the length of one of the sides of said surface to be exposed, said means for processing the light beam comprising a reflector and an integrator-collimator assembly;
displacement means for generating relative displacement between said light strip and said face to be exposed in a direction substantially transverse to the longitudinal direction of said light strip; and
matching means for matching the speed of relative displacement between said light strip and said face to be exposed to the illumination of the light strip and to the sensitivity of the surface to be exposed.
In the usual case of panels that are substantially rectangular, it will be understood that scanning is accelerated by being performed in a direction parallel to the width of the panel, i.e. when the light strip is parallel to the long direction of the panel. In this first circumstance, the length of the light strip is not less than the length of the long side of the panel and scanning takes place in the short side direction.
For reasons of size, it can be necessary to turn the panel relative to that first configuration and scan it in its long direction. In this second configuration, the length of the light strip is not less than the width of the panel and scanning takes place in the long side direction.
Advantageously, all the optical system is stationary so that the optical means contained therein are not subjected to unwanted loss of adjustment that could degrade the uniform and collimated light beam that leaves said assembly.
At the inlet to the integrator-collimator assembly placed after the light source, the light is neither collimated nor distributed in uniform manner, whereas at the outlet therefrom it is distributed uniformly, e.g. with an error of less than xc2x110% for a light strip measuring 780 millimeters (mm) by 170 mm, and it is collimated with a mean angle of incidence of less than 2xc2x0, and preferably less than 1xc2x0.
Advantageously, the integrator-collimator assembly comprises a first optical processor unit for spreading the light in substantially uniform manner and a second optical processor unit, said second unit being placed after said first unit and serving to collimate the light.
For reasons of cost and feasibility, each optical processor unit performs a specific function (either collimation and declination or else homogenization).
It will thus be understood that the characteristics of the optical processor unit and their respective dispositions determine the properties of the light beam obtained at the outlet from the integrator-collimator assembly.
Thus, advantageously, the first optical processor unit is placed firstly at a second focus of said reflector so that said first optical processor unit can process said light beam entering therein to deliver an outlet light beam that is uniform, and secondly at the object focus of said second optical processor unit so that said second unit can process the uniform light beam entering it to produce a light beam that is uniform and collimated.
Since the light is uniform, variation in exposure power at any point of the light strip is controlled and does not exceed extreme values that would run the risk of damaging the tracks. Similarly, since the light is collimated, the light rays are mutually parallel and all of them arrive at an angle of incidence smaller than 2xc2x0.
The integrator-collimator assembly also advantageously comprises a first mask placed in the vicinity of said first optical unit and a second mask placed in the vicinity of said second optical unit.
The first mask serves to eliminate a fraction of the (non-uniform, non-collimated) light radiation having a mean angle of incidence that is too far removed from the mean propagation direction at the outlet from the reflector, while the second mask has the same function for the uniform light beam that has passed through the first optical processor unit. These two masks perform coarse collimation by removing rays that diverge excessively, and by doing so on two successive occasions prior to the light entering the second optical unit that serves to perform collimation. Collimation is thus more effective in that the diverging rays have already been eliminated.
The temperature of the artwork is an important factor in determining the quality with which the printed circuit is made since any temperature gradient leads to the artwork becoming deformed, and thus to deformation in the paths followed by the tracks. For example, a temperature difference of as little as 2xc2x0 C. can lead to image distortion.
Thus, in order to minimize temperature variations in the artwork, the apparatus advantageously includes a dichroic mirror.
Infrared radiation is not useful for exposure but it does heat adjacent elements, and in particular the artwork, so it is advantageous for the anti-heat filter to separate the light beam into infrared and ultraviolet portions, and then for the panel to be exposed with xe2x80x9ccoldxe2x80x9d light, i.e. light that is essentially ultraviolet.
The shaper means advantageously comprise a first mirror which is diverging and convex and a second mirror which is converging and concave, the mirrors being placed in succession at the outlet from said integrator-collimator assembly.
At the inlet to the shaper means, the light beam is uniform and collimated, but it is still not in the form of a light strip but is generally in the form of a two-dimensional rectangle that is much smaller in size than the size desired for scanning along one of the sides of the panel. The first mirror serves to de-collimate the light beam in the plane of the mirror by causing it to diverge in one of the two dimensions so as to spread out the light beam into a strip of light of length greater than its initial length.
The second mirror serves to re-collimate the light beam in the same plane by making it converge in the same dimension so that the light strip has collimation properties that are identical to those of the light beam entering the shaper system.
The uniformity of the light beam and thus of the light strip remains unchanged throughout shaping.
The positions of the mirrors relative to the integrator-collimator assembly has no influence on the properties of the light strip, but it will be understood that their positions relative to each other, and in particular the spacing between them and the characteristics of said mirrors determine the shape of the light strip.
Thus, the length of said uniform and collimated light strip is advantageously a function of the spacing between said converging and diverging mirrors, and of the radii of curvature of said mirrors, and in particular the radius of curvature of the diverging mirror. The height of the light strip depends on the geometrical characteristics of the collimating lens and of its support. The height of the light strip is affected very little by the converging and diverging mirrors.
The displacement means advantageously comprise a plane mirror that is movable in the plane defined by the axes of the face of the panel to be exposed.
This mirror has no effect on the properties of the light strip and it is placed at the outlet of the optical system that has served to process and to shape the light beam. The mirror is the only moving part of the apparatus and it enables the entire surface to be exposed in succession by scanning, reflecting a light strip that moves in translation towards the panel to be exposed.
Naturally, the dimensions of the mirror are matched to the desired length of the light strip so as to avoid unwanted shortening of said length.
Since the shape of the light strip is determined directly by the selected shaper optical system it is easily modified.
To obtain better power uniformity, the light source is advantageously placed at a first focus of the reflector. When using an arc lamp, it is preferably the arc which is placed at the first focus of the reflector.
Thus, a very large fraction of the light emitted by the light source is reflected towards the second focus of the reflector which corresponds to the inlet to the processor and shaper system constituted by the first optical processor unit for processing and shaping.
In a second aspect, the invention provides apparatus for improving exposure of a surface, in particular for manufacturing printed circuits, by using a moving light source.
In this second aspect, the invention provides apparatus comprising:
means for holding at least one artwork and said panel on a frame;
an optical system comprising at least one light box containing a light source and a reflector comprising at least a first parabola having a first focus and a second parabola having a second focus, said first and second focuses being situated on the axis corresponding to a direction for scanning said face to be exposed, the light source being placed at said first focus to generate a light strip on the surface of the panel to be exposed and including said artwork, the mean angle of incidence of the light strip being less than 15xc2x0 and the uniformity of its illumination presenting departures of less than xc2x110% relative to a mean value, said light strip being of length not less than the width of said surface to be exposed;
displacement means for generating relative displacement between said light strip and said face to be exposed in the length direction of said face to be exposed, which direction is substantially transverse to the longitudinal direction of said light strip; and
matching means for matching the relative displacement speed between said light strip and said face to be exposed to the illumination of the light strip and to the sensitivity of the surface to be exposed.
The specific shape of the reflector determines the properties and shape of the light beam.
Advantageously, said first parabola is situated in a first plane defined by axes including said scanning direction, and said second parabola is situated in a second plane defined by the axes including said scanning direction and substantially transversal to said first plane.
To make such a reflector easier to manufacture and to reduce costs, the reflector is advantageously built up from two symmetrical portions. In addition, to make it easier to change the lamp, the reflector advantageously includes a central opening.
In order to obtain a light strip measuring approximately 635xc3x97130 mm2, the optical system advantageously comprises five light boxes in alignment along the direction extending transversely to said scanning direction, which boxes are moved in translation in the scanning direction, i.e. in a direction that is substantially transversal to the length of the light strip so as to scan the entire panel. In this optical configuration, the mean angle of incidence obtained with five medium arc lamps is about 11xc2x0. Medium arc lamps make it more difficult to collimate the beam, but they make it easier to superpose the light sources.
Naturally, in order to obtain a light strip of different shape, it is appropriate to adjust the number of light boxes and their light-emitting power, and also their respective arc lengths, and possibly also to modify the reflector.
The spacing between the boxes is preferably 145 mm, with it being possible to perform adjustments in the longitudinal direction of the light strip in order to compensate for inaccuracies in light box manufacture.
In both of the above aspects of the invention and at any given instant, the collimated light strip advantageously forms a quadrilateral on the surface to be exposed, having height lying in the range 100 mm to 150 mm and length that is not less than the length of one of the sides of the surface to be exposed.
In order to generate the light strip, said light source advantageously comprises a medium or short arc electric discharge lamp.
In general, short arcs have a length of less than 10 mm and medium arcs have a length lying in the range 10 mm to 25 mm. Beyond 25 mm, the lamp is said to have a xe2x80x9clongxe2x80x9d arc. The shorter the arc of the lamp, the better the collimation; however, short arc lamps require a more sophisticated power supply.
Thus, depending on the type of lamp and the type of optical system selected, the light strip advantageously presents a mean angle of incidence that is less than or equal to 2xc2x0 or 15xc2x0.
The apparatus advantageously further comprises calibration means for calibrating the light source or for calibrating each of the light sources independently of one another when there are several light sources.
In the presence of a plurality of light sources, the calibration means advantageously comprise a single sensor which is moved successively in front of each light source. On the basis of the light intensity measured for one source, the signal from the sensor acts via a servo-control loop to control power supply regulation for the corresponding light source, and thus to control the light power emitted thereby.
Once the power of each light source has been adjusted, uniform power is obtained over the light strip. Given the mean value of this power and the nature of the surface to be exposed, a computer in the apparatus adjusts scanning of the light strip, matching the travel speed of the light strip to the light power and to the nature of the surface to be exposed, so as to obtain the desired exposure conditions.